The present invention relates to an integrated pumping and/or energy recovery system. Typically, such systems are used to pump an input stream of fluid to be purified through a membrane or filter, such as a reverse osmosis membrane, at a high pressure. A stream of brine or other unpurified material is then discharged under pressure from such membrane or filter. In a preferred application, the present invention recovers energy from the discharge stream while it is still under pressure, and then uses such recovered energy for a useful purpose, e.g., to reduce the amount of energy that the pump would otherwise have to expend in order to pump the input stream of fluid into the system, thereby making operation of the purification system more efficient. An example of a system with which the present invention may be used is a reverse osmosis (RO) system.
As is known in the art, when a semi-permeable membrane divides two fluids of different salinities, osmosis occurs. To achieve equilibrium of the chemical potential across the membrane, liquid flows through the membrane into the more concentrated solution. This flow continues until concentrations on either side of the membrane are equal, unless the osmotic pressure is reached. The osmotic pressure, when reached, functions as a static head (due to the rising level on one side of the membrane) resulting in zero flow through the membrane.
A pressure that is applied in addition to the osmotic pressure causes the osmosis to reverse. With the flow reversed, liquid (referred to as permeate) flows from the more concentrated side of the membrane. Thus, practical systems may be designed with an operating pressure above the osmotic pressure in order to force a reverse osmotic fluid flow from the more concentrated side of the membrane to a desalted permeate. In this manner, desalted permeate may be produced from a saline feed stream. That is, reverse osmosis desalination systems may be designed that desalt brackish or seawater.
The osmotic pressure is a colligative property of the fluids being processed and is dependent on the concentrations of salts and minerals in the fluid. For seawater, the osmotic pressure is approximately 25 kg/cm.sup.2, or approximately 355 pounds per square inch (psi). Presently commercial membranes operate at approximately 55-82 kg/cm.sup.2 (approximately 800-1200 psi). Of course, limits exist on the permissible concentration of salts within the membranes. Supersaturation of salts may result in deposition of salts on the membrane, and increased concentrations have higher osmotic pressures. Commercial designs typically provide a permeate flow of 30-55% of the input feed liquid (the seawater), which permeate flow is commonly referred to as the Recovery Rate or Recovery Ratio of the reverse osmosis system.
It is known in the art to improve the efficiency of the reverse osmosis process by recovering energy from the high pressure waste brine. Known methods of pumping and of energy recovery include, for example, some combination of: plunger pumps with belt drives and pulsation dampeners, centrifugal pumps, sumps and sump pumps, reverse flow pump and Pelton wheel energy recovery turbines, hydraulic turbo chargers, flow work exchangers, and variable frequency drives.
The present invention accomplishes the same functions achieved by the above-listed prior art pumping and energy recovery methods in a unique way. The energy recovery function of the present invention most closely resembles an energy recovery system of the work exchanger type. Numerous types of work exchanger energy recovery systems have been proposed for use with reverse osmosis or similar systems. See, e.g., U.S. Pat. Nos. 3,558,242; 3,791,768; 3,825,122; 4,124,488; Re 32,144; 4,410,429; 4,432,876; 4,434,056; Re 33,135; 4,637,783; 4,756,830; 4,830,583; 4,836,924; 5,306,428 and 5,500,113.
Of these prior art energy recovery systems, two are of particular interest relative to the invention disclosed herein. A first type, which will be referred to hereafter as a flow work exchanger (FWE) system, is illustrated, e.g., in U.S. Pat. No. 5,306,428. In a FWE system, energy is recovered from waste brine streams through the use of sliding pistons which pressurize the feed stream, thereby reducing the amount of energy that the main pump must expend in order to raise the feed stream to the desired pressure for reverse osmosis operation. Such systems typically operate at a very slow piston cycle rate, require valve switching at full flow, and thus cause enormous pressure pulsations to occur, all of which contribute to significant maintenance costs and a relatively short component lifetime.
A second work exchanger type of energy recovery system of interest to the present invention will be referred to hereafter as a "bang bang" system. In general, a "bang bang" system employs pistons of different areas with connecting rods. An example of a bang bang system is shown in U.S. Pat. No. 3,825,122. In a "bang bang" system, energy is recovered from waste brine streams through the use of an energy recovery piston that is mechanically linked with the main reciprocating piston of the pump. Thus, as the pump piston moves in a forward stroke direction, pressure is applied through the energy recovery piston, which is connected through appropriate valves with the waste brine stream, to aid in pushing the pump piston along in its forward direction. When the piston reaches the end of its forward direction, it bangs into the end of its stroke (hence, the term "bang bang") and prepares for movement in the other direction. When the pump piston changes direction, the setting of the energy recovery valves must be altered so that the added force supplied through the energy recovery piston is applied in the reverse direction. Unfortunately, as with the FWE system, the valves must be switched at full or near-full flow, thereby creating significant pressure fluctuations in the system. Moreover, the pressure differential across the pump energy recovery piston, as well as the main pump piston, remains high at all times, equal to or near the full pressure of the system, thereby magnifying maintenance problems associated with sealing such pistons, and compromising the life of the pump components.
It is thus apparent that what is needed is an integrated pumping and energy recovery system that accomplishes the functions provided by the previously-mentioned devices and methods, yet overcomes the shortcomings of the FWE and bang-bang type systems. The present invention addresses these and other needs.